Transgenic mice expressing the human poliovirus-receptor protein have been compared with rhesus monkeys in intraspinal tests for neurovirulence of oral poliovirus vaccines (OPV). Results from previous years were summarized at a WHO Meeting at the end of FY95 and recommendations of that meeting instituted this year. A procedure manual was prepared, accepted by WHO and distributed to all participants in Collaborative Study. New participants were trained, and several successfully confirmed test results of LMD, discriminating between a reference vaccine and one with elevated monkey neurovirulence. The optimized Tg-mouse test also discriminated consistently between fully attenuated type-3 OPV and revertant OPV vaccines that have only slightly increased neurovirulence for monkeys; those results were confirmed by two experienced participants in the WHO study. Preliminary tests in LMD successfully discriminated between fully attenuated and revertant type-2 OPV vaccines.